In power plants, it is necessary to monitor the concentration of soluble silica compounds in the water fed to and in the boilers. Excessive amounts of such compounds (referred to hereinafter as "silica") can cause coating of the turbine blades which requires costly maintenance on a periodic basis. Typically, silica concentrations in excess of 20 parts per billion (ppb) are considered excessive.
Analyzers have been sold for continuously monitoring the concentration of silica and other chemical compositions in water and other fluids. One such device was manufactured and sold by Orion Scientific Instruments of Hawthorne, N.Y. as the Orion Model 1830 Silica Analyzer (hereinafter referred to as the Orion analyzer).
The Orion analyzer coupled a sample of water to be analyzed to a chemical cartridge where known reagents were added and mixed in proper sequence to yield a heteropoly blue complex, the intensity of which was proportional to the silica concentration. The heteropoly blue complex was then pumped to a flow cell positioned in a colorimeter of the type shown in U.S. Pat. No. 4,273,449 of Schmid entitled "Radiation Measuring Apparatus".
This colorimeter directed light through the flow cell and through a reference path so that adjustments for changes in the optical signals could be accommodated. In accordance with known procedures, a baseline value (corresponding to a zero silica content) and a full scale value could be entered into storage in a microprocessor which would then calculate a calibration curve. These data entries were made manually by the operator who would select the switching of a baseline solution and a standard solution to the chemical cartridge and flow cell for the necessary measurements. After the analyzer had been calibrated the operator would then connect the sample to the analyzer and the analyzer would then be able to compare the colorimetric values, after the appropriate reactions, with the calibration curve in the memory of the microprocessor.
The Orion analyzer had a number of drawbacks. In the first place, the procedure was not automatic and a fairly high level of skill was required to prepare and calibrate the analyzer for the sample measurements. For example, the reflecting surfaces of a prism used to divide the beam between the reference cell and flow cell paths had to be initially adjusted so that the two detectors produced essentially the same output at the start. This was necessary because the detectors were part of a ratio system. Errors in the calibration and setup procedures would be reflected in the readings of the analyzer while monitoring the sample to be tested.
Furthermore, the colorimetric system used in the Orion analyzer and as shown in the '449 patent required a flow cell having a relatively long path length to provide adequate sensitivities for very low concentrations. This was caused in part by the need to use a beam divider to separate the light into measurement and reference paths. Reflections caused by the beam divider result in a loss of light energy (i.e., the beam divider is less efficient than a "straight through system"). Thus, when low signal levels or relatively low "delta" signals are processed, the signal-to-noise level becomes a problem in detecting low concentration levels. The beam divider system thus requires the use of a long flow cell light path to compensate for a very low energy difference between the baseline solution and the absorption where the concentration of the parameter being measured is very low. However, increasing the length of the flow cell increases the tendency of air bubbles to collect in the cell. This gives rise to a serious problem. The stream within the analyzer comprise pockets of fluid separated by air bubbles. These air bubbles are discarded before the fluid enters the flow cell. However, the presence of entrapped air bubbles in the flow cell of the colorimetric analyzer, even if microscopic in size, affects the transmission of light through the cell and, therefore, is likely to cause false readings and noisy recordings. The type of flow cell commonly used today for continuous colorimetric analysis does not lend itself to reliable venting of all air within the samples under test.